1. Field of the Invention
The present invention is related to an illuminating system and method for illuminating a pattern generator, and more particularly for illuminating a pattern generator in a lithography system.
2. Background Art
Pattern generators are used in many different environments to pattern objects or project patterns using light, for example, in lithography systems, televisions, biomedical systems, biotechnology systems, etc. Typically, reticles (or masks), spatial light modulators (SLMs) or contrast devices (hereinafter, both are referred to as SLMs), such as digital mirror devices (DMDs), liquid crystal displays (LCDs), grating light valves (GLVs), or the like, or any other elements that include a transmissive and/or reflective pattern can be used as pattern generators.
SLMs can include an active area having an n×m (wherein n and m are integers greater than 1) array of active devices (or pixels) (e.g., an array of mirrors on the DMD, an array of gratings on a GLV, or an array of reflective/transmissive devices on the LCD). Each active device is individually controlled to move the active devices between ON and OFF through one or more discrete states. For example, if the active devices are mirrors on the DMD, each of the mirrors is individually controlled to rotate or tilt the mirror to either binary or multiple positions. As another example, if the active devices are strips in a GLV, sets of strips can be bent or straight to allow reflection or diffraction of incoming light beams.
It is to be appreciated that controlling the active devices in active areas so that they are partially or fully ON or OFF is well know in the art, and not fully described here for brevity. Typically, a predetermined and previously stored algorithm based on a desired exposure pattern is used to turn ON (or partially ON) and OFF the active devices, as is known in the relevant arts.
FIGS. 1, 2, and 3 show conventional systems 100, 200, and 300, respectively, for illuminating a pattern generator, so that patterned light is formed and directed from the pattern generator. As is known, the illumination optics, and optional pattern generator optics, can include one or more optical elements (e.g., lenses, mirrors, etc.). In one arrangement, the illumination optics can include the pattern generator optics. In another arrangement, the pattern generator optics can be a separate element. A projection system would normally focus patterned light from the pattern generator onto a substrate.
FIG. 4 shows a convention illumination field 400 that can result from systems 100, 200, and/or 300 for pattern generator 402 having desired illumination areas 404. Each illumination area can be either an active area of an SLM or a desired portion of a pattern on a reticle. As discussed above, each active area will include the active devices. As can be seen, illumination field 400 is so large that it not only impinges on desired illumination areas 404, but is larger than pattern generator 402. Thus, a substantial amount, maybe up to about 80–90%, of the light may be wasted (i.e., not used during operation of system 100, 200, and/or 300) because that amount of light does not impinge on desired illumination areas 404.
One use for the pattern generator, or an array thereof, is in maskless lithography. Lithography is a process used to create features on the surface of a substrate. Such substrates can include those used in the manufacture of flat panel displays (e.g., liquid crystal displays), circuit boards, various integrated circuits, and the like. A frequently used substrate for such applications is a semiconductor wafer or flat panel display glass substrate. While this description is written in terms of a semiconductor wafer for illustrative purposes, one skilled in the art would recognize that this description also applies to other types of substrates known to those skilled in the art.
During lithography, a wafer, which is disposed on a wafer stage, is exposed to an image (e.g., a pattern) formed by the pattern generator, or array thereof. The image is projected onto the surface of the wafer by exposure optics located within a lithography apparatus. While exposure optics are used in the case of photolithography, a different type of exposure apparatus can be used depending on the particular application. For example, an excimer laser, x-ray, ion, electron, or photon lithography can each require a different exposure apparatus, as is known to those skilled in the art. The particular example of photolithography is discussed here for illustrative purposes only.
The projected image produces changes in the characteristics of a layer (e.g., photoresist) deposited on the surface of the wafer. These changes correspond to features in the image projected onto the wafer during exposure. Subsequent to exposure, the layer can be etched to produce a patterned layer. The pattern corresponds to the features projected onto the wafer during exposure. This patterned layer is then used to remove or further process exposed portions of underlying structural layers within the wafer, such as conductive, semiconductive, or insulative layers. This process is then repeated, together with other steps, until the desired features have been formed on the surface, or in various layers, of the wafer.
Step-and-scan technology works in conjunction with a projection optics system that has a narrow imaging slot. Rather than expose the entire wafer at one time with the image formed by the pattern generator, individual fields are scanned onto the wafer one at a time. This is accomplished by moving the wafer and controlling active devices on the pattern generator, such that the imaging slot is moved across the field during the scan. The wafer stage must then be stepped between field exposures to allow multiple copies of the pattern formed by the active devices on the pattern generator to be exposed over the wafer surface. In this manner, the quality of the image projected onto the wafer is maximized.
Desired illumination areas on a pattern of a reticle (or mask) or the active area of the SLM are usually substantially smaller than a size of a surface incorporating the desired illumination areas of the pattern or the active area. For example, in an SLM, an active area may only be 10–20% of the SLM surface, with the remaining surface area of the SLM being an inactive area, which can include packaging, circuitry, and the like. Thus, a substantial amount of the light directed to the pattern generator may not impinge on the desired illumination area of the pattern or the active area, but instead impinges on undesired areas of the pattern or the inactive areas, which can result in stray light and/or wasted light. Some of the stray light can reach the surface of the substrate. The stray light reaching the surface of the substrate can cause errors in devices being fabricated on the substrate.
Therefore, what is needed is a system and method that increases optical efficiency by directing light such that it substantially impinges on desired illumination areas of a pattern generator and that reduces or substantially eliminates stray light caused from light impinging on undesired areas of the pattern generator.